Nicholas Harris, 45, Mark Dixon, 43, and Matthew Higgins, 33, were convicted of conspiracy to defraud at Bristol Crown Court.
The trio took deposits from their victims but never completed a single job.
The cowboys set up two firms – Construction Management Development Ltd and CMD Construction Services Ltd – to carry out the fraud.
Dixon, of Ash Lane, Down Hatherley; Harris, of Ash Path, Upton St Leonards and Higgins, of Nelson Street, Gloucester, had all denied conspiracy to defraud between April 2005 and October 2007.
Two other men, Leighton Docksey, 35, of Bathurst Road, Gloucester, and Lee Ireson, 30, of Gloucester Road, Cheltenham, were cleared of fraud charges.
The guilty men pretended that their firms were large UK-wide contractors and collected as much cash as possible up-front before stalling over the start of work.
A couple who fell victim to the Gloucester gang told the BBC they were “well and truly conned”.
Susan Giesbrecht and her husband Ben were left £20,000 out of pocket when CMD Services failed to build an extension to their home.
Mrs Giesbrecht, from Down Ampney, near Cirencester, said: “They sent along an incredibly plausible young man who got very friendly with us and seemed very amiable. We were completely taken in by him.
“I think both my husband and I would assume that because we were intelligent people, that we were unlikely to be duped by anybody. And we were well and truly conned by these people.”
Mr Giesbrecht said: “As I took more control of the project and started to put pressure on CMD to either do some work or to give us some money back, their reaction became increasingly hostile and aggressive.
“This included threats of legal action; I started receiving obscene nuisance phone calls and their e-mail correspondence became more and more aggressive.”
Businessman Mike Stanley, from Yate in South Gloucestershire, paid CMD Services £11,000 for an extension to his home.
After endless delays and excuses by the builders he told the company he was contacting the police.
He said: “Then I got a phone call back from this person who turned out to be Nick Harris saying that we should never threaten them with the police, if I do any more of this I’ll be breaking the contract and they therefore wouldn’t do any more work.
“I said ‘you’ve hardly done any work as it is’ and then things got pretty nasty on the telephone.
“I’m not that old but how they can go out and con pensioners out of their life savings I don’t know.
“It’s just despicable, I just hate them.”
He said he eventually had to spend another £11,000 to complete the work.
The three guilty men will face sentence on a date yet to be fixed.
